The use of water injection or waterflood operations to recover oil from subterranean reservoirs is a well known and commonly employed practice in the petroleum industry. A typical waterflood comprises injecting an aqueous flooding medium, e.g. water, into the reservoir to drive oil through the reservoir toward one or more production wells from which it can be collected at the surface of the earth. Unfortunately, the injected water tends to channel through certain portions of the reservoir, while completely bypassing other portions of the reservoir. This inability of water to sweep a substantial percentage of the volume of the reservoir within the pattern of the wells employed in the waterflood operation seriously affects the ultimate recovery of oil, and detracts from the economic attractiveness of the operation. Accordingly, there is a need to improve the sweep efficiency of waterfloods and this need has long been recognized by persons working within the oil industry.
It is taught in the art that poor sweep efficiency is a result of several factors. One is the natural tendency of the liquid to flow in the path of least resistance; hence, water flows through the highly permeable portions of the heterogeneous reservoir more readily than through the less permeable portions. Another factor involves the difference between the mobilities of the injected water and the oil present in the reservoir. In both instances, sweep efficiency of a reservoir can be improved by increasing the viscosity of the injected water.
The use of hydrophilic, viscosity-increasing additives for flood water is known and practiced in the art, and commonly employed additive for this purpose include partially hydrolyzed polyacrylamides, copolymers of acrylamide and acrylates, and one of the very promising groups of thickeners, ionic polysaccharides, particularly the polysaccharide prepared by employing bacteria of the genus Xanthomonas and commonly referred to as polysaccharide B-1459.
While polysaccharides exhibit many advantageous performance characteristics, and are preferred over other hydrophilic polymers in many applications, certain problems have been identified which limit their effectiveness, at least in certain reservoirs. The most effective and desirable polysaccharides from the standpoint of developing high viscosity at relatively low concentration levels, are somewhat difficult to disperse completely in relatively saline environments, e.g. in field brines containing more than about 50,000 parts per million total dissolved solids. Additionally, problems are known to exist in connection with polysaccharides which manifest themselves as loss of viscosity over long periods of time. Oxidative degradation was identified early in the course of examining these compounds, and it is now common practice to disperse polysaccharides in low oxygen-containing water, and to further protect them again oxidative degradation by incorporating an oxygen scavenger in the fluid. Two unexpected problems have been identified, which are encountered whenever the fluids are prepared in, or when the fluids are brought into contact with, water containing appreciable concentrations of water soluble borates or other boron containing compounds. The first problem is especially severe when the polysaccharide contains at least ten percent D-mannose, and the above-identified polysaccharide contains 33 percent D-mannose. When fluids containing fully hydrated and dispersed polysaccharides which are comprised of at least 10 percent mannose, contact water containing in excess of 7.5 parts per million borates (as boron), precipitation occurs which is manifest in significant loss of fluid viscosity. The reaction is not instantaneous, but occurs slowly after contact between polysaccharide and borate has continued for more than about 30 days at temperatures above 120.degree. F. Since the viscosity loss is precisely the same symptom as results from oxidative degradation of the polysaccharide, it is not immediately apparent to those employing these fluids which phenomena is responsible for the observed loss in viscosity of solution. A second, possibly related problem is concerned with injectivity problems. Problems such as plugging, especially in tight or low permeability formations, are encountered whenever the fluid is prepared by initially hydrating the polysaccharide in water containing more than about 1.7 ppm boron.
In view of the foregoing discussion, it can be appreciated that there is a serious, unfulfilled need for a method for preparing an aqueous, hydrophilic polymer-containing fluid, particularly a fluid containing polysaccharides such as polysaccharide B-1459, or any polysaccharide containing sufficient mannose to cause it to be sensitive to borates, so as to eliminate or reduce the sensitivity of the polysaccharide fluid to subsequent contact with borates, and further to ensure that the fluid may be easily injected into a formation without danger of plugging small flow channels.